RS232 Cables, Wiring and Pinouts

RS232 Wiring Stuff

RS232 standards are defined by EIA/TIA (Electronic Industries Alliance /Telecommunications Industry Association). RS232 defines both the physical and electrical characteristics of the interface. RS232 is practically identical to ITU V.24 (signal description and names) and V.28 (electrical). RS232 is an Active LOW voltage driven interface and operates at +12V to -12V where:

Signal = 0 (LOW) > +3.0V

Signal = 1 (HIGH) < -3.0V

Notes:

1. Signal voltages in the range >-3.0V to +3.0V is regarded as the 'dead area' and allows for absorption of noise. For more on this stuff go here.

2. The power level on RS232 pins is defined by TIA in terms of short circuit protection at 100mA. Most RS232 drivers will provide lower short circuit protection (especially for laptops). A max of 50mA PER PIN may be available but the data sheet for the specific interface should consulted before commiting to externally powered designs.

3. We received an email recently pointing out some issues with NULL modem cables. The pinout shown below will gnerally work. However there are many permutations of signal sets that can be used by either end of a connection and they may not be SYMETRIC. One end may expect something (a signal) that the other end cannot generate. This typically happens withCTS/RTS (and perhaps DCD) and DTR/DSR. If you suspect this is the case then unfortunately you need to UNDERSTAND the interface and may have to SPOOF certain signals. Our signal primer page may help you. Finally if you are having serious problems splash out on a light box or some other device that will show you which signals are being activated.

4. Watch the terms DTE (Data Terminal Equipment - e.g. a terminal or PC) and DCE (Data communications Equipment - e.g. a modem) the meaning and use of certain pins may differ. All the diagrams below define the interface from the DTE perspective. An RS232 interface has a female and male connector, the male connector has the bits sticking out!!

5. The terms Data Carrier Detect (DCD) and Received Line Signal Detect (RLSD) are one and the same. We use DCD throughout 'cos we think its more common.

6. Like most folks we use the term DB9 which is widely - but erroneously - used to describe a 9-pin serial connector. We got an email pointing out the error of our ways. So, if you want to amaze your friends over the dinner table you can read more here and use the technically correct terms in the future. Seriously, while we get away with it most of the time (with common or garden PCs) sometimes its essential to know EXACTLY what connector type you are talking about.

7. RS-232-E is normally defined to be used with a DB25 connector, but does have a 26 pin alternate (and much smaller connector). We suggest that if you come across one of these that you do the decent thing - use an expletive (alternatively with your luck you could consider buying a lottery ticket)!

8. We have received a number of emails recently asking how to wire DB9's using cat5(e) cable. We guess there is lot more LAN cable lying around these days. We have added a null modem only section to cover this wiring. There is absolutely no standard to cover this form of wiring. This section is simply offered as one of many possible ways to do it.

Contents

RS232 on DB25 Pinout (RS-232C)

RS232 on DB9 Pinout (EIA/TIA - 574)

RS232 on RJ45 (RS-232D EIA/TIA-561)

RS232 DB25 NULL Modem Pinout

RS232 DB9 NULL Modem Pinout

RS232 DB9 NULL Modem Pinout using Cat5(e)

RS232 DB9 to DB25 Pinout

RS232 DB9 to DB25 NULL Modem Pinout

V.35 on a DB25 (EIA/TIA RS-530-A)

RS232 on DB25 (RS-232C)

Signal/pin primer

Note: This is NOT the same as the DB25 Parallel port on a PC which is defined here.

Pin No.	Name	Notes/Description
1	-	Protective/shielded ground
2	TD	Transmit Data (a.k.a TxD, Tx)
3	RD	Receive Data (a.k.a RxD, Rx)
4	RTS	Request To Send
5	CTS	Clear To Send
6	DSR	Data Set Ready
7	SGND	Signal Ground
8	CD	Carrier Detect (a.k.a DCD)
9	-	Reserved for data set testing
10	-	Reserved for data set testing
11	-	Unassigned
12	SDCD	Secondary Carrier Detect
13	SCTS	Secondary Clear to send
14	STD	Secondary Transmit Data
15	DB	Transmit Clock (a.k.a TCLK, TxCLK)
16	SRD	Secondary Receive Data
17	DD	Receive Clock (a.k.a. RCLK)
18	LL	Local Loopback
19	SRTS	Secondary Request to Send
20	DTR	Data Terminal Ready
21	RL/SQ	Signal Quality Detector/Remote loopback
22	RI	Ring Indicator (DCE raises when incoming call detected used for auto answer applications)
23	CH/CI	Signal Rate selector
24	DA	Auxiliary Clock (a.k.a. ACLK)
25	-	Unassigned

NOTE: Leave all pins not specified above unconnected.

view - looking into male connector

RS232 on DB9 (EIA/TIA 574)

Signal/pin primer

Pin No.	Name	Notes/Description
1	DCD	Data Carrier Detect
2	RD	Receive Data (a.k.a RxD, Rx)
3	TD	Transmit Data (a.k.a TxD, Tx)
4	DTR	Data Terminal Ready
5	SGND	Ground
6	DSR	Data Set Ready
7	RTS	Request To Send
8	CTS	Clear To Send
9	RI	Ring Indicator

View - looking into male connector

RS232 on RJ45 (RS-232D)

More properly EIA/TIA - 561. Use when connecting to or from a serial port with a 8 position Modular Jack (RJ45). If you are cross-connecting from a DB9 or a DB25 use the signal names to cross connect the appropriate connections.

Signal/pin primer

Pin No.	Name	Notes/Description
1	DSR/RI	Data set Ready/ring indicator
2	DCD	Data Carrier Detect
3	DTR	Data Terminal Ready
4	SGND	Signal Ground
5	RD	Receive Data
6	TD	Transmit Data
7	CTS	Clear to Send
8	RTS	Request to Send

Note: Pin 1 is a multi-function pin sharing with DSR (Data Set Ready) and RI (Ring Indicator). This means it is impossible to differentiate between a incoming ring signal and when the modem has finally connected and synched up. With local (null modem connections) or if the modem is run in auto-answer mode this is not normally a problem. If used with a modem and the DTE (the computer end) wants to control the connection the problem is more real. DSR would normally indicate the 'connected and synched-up' state following DTR from the DTE. DCD will indicate that a carrier has been received but does not indicate synchronization of both ends. In most cases however CLS (Clear To Send) in response to RTS (Request To Send) will not normally be returned until an end-to-end connection is available.

RJ45 Male Connector Pin Numbering

RS232 DB25 NULL Modem Pinout

Use when connecting two systems (e.g. PCs) via their DB25 interfaces without a modem (i.e. back-to-back). See the full signal names in the DB25 sections.

If this pinout does not work for you then you could try our Signal/pin primer because you may need to SPOOF connections.

Note: This DB25 is NOT the same as the DB25 Parallel port on a PC which is defined here.

DB25	Signal	DB25	Signal
3	RD	2	TD
2	TD	3	RD
20	DTR	6,8	DSR, DCD
6,8	DSR, DCD	20	DTR
4	RTS	5	CTS
5	CTS	4	RTS
7	SGND	7	SGND
22	RI	22	RI

NOTE:

1. Leave all pins not specified above unconnected.

2. We have received email suggesting that the above pinout looks like DTR from one side is driving into DSR/DCD on the other side - not normally a healthy situation. The emails miss the point that since both ends are DTEs NEITHER should be attempting to drive the DSR/DCD signals. They are essentialy RX only signals on both sides.

RS232 DB9 NULL Modem Pinout

Use when connecting two systems (e.g. PCs) via their DB9 interfaces without a modem (i.e. back-to-back). See the full signal names in the DB9 section.

If this pinout does not work for you then you could try our Signal/pin primer because you may need to SPOOF connections.

DB9	Signal	DB9	Signal
2	RD	3	TD
3	TD	2	RD
4	DTR	6,1	DSR, DCD
6,1	DSR, DCD	4	DTR
7	RTS	8	CTS
8	CTS	7	RTS
5	SGND	5	SGND
9	RI	9	RI

NOTE:

1. We have received email suggesting that the above pinout looks like DTR from one side is driving into DSR/DCD on the other side - not normally a healthy situation. The emails miss the point that since both ends are DTEs NEITHER should be attempting to drive the DSR/DCD signals. They are essentialy RX only signals on both sides.

RS232 DB9 NULL Modem Pinout on CAT5

This is in response to a number of recent emails asking how to wire both ends of a DB9 connection using cat5(e) cable. This must not be confused with DB9 to RJ45 (RS232D). We have shown a null modem (back-to-back PCs) only configuration. And if you want to use cat5(e) with a real modem? Our advice - don't.

Warning:. There is no standard to cover the use of cat5(e) (8 conductor) wiring when used with two DB9 connectors. Any wiring scheme is non-standard - that includes the wiring scheme below. Specifically this means that both ends of the cable must be wired in the same way and that no assumptions can be made about how the other end is wired. You will have to manually inspect both ends of the connection. Damage can result from mis-matched wiring.

A DB9 clearly has 9 connections and a cat5(e) cable has 8 conductors. RS232D has chosen to use Pin 1 as a multi-function pin (DSR/RI) to provide maximum flexibility with modems. We have chosen to use a minor variation on the normal DB9 Null modem pinout above. The actual colors are unimportant but the suggested configuration is one way to provide the shortest use of the adjacent (twisted) pairs.

If this pinout does not work for you then you could try our Signal/pin primer because you may need to SPOOF connections.

DB9	Signal	cat5(e) Color	DB9	Signal	cat5(e) Color
2	RD	Brown	3	TD	Blue
3	TD	Blue	2	RD	Brown
4	DTR	Green	6,1	DSR, DCD	Brown-white
6,1	DSR, DCD	Brown-white	4	DTR	Green
7	RTS	Blue-white	8	CTS	Green-white
8	CTS	Green-white	7	RTS	Blue-white
5	SGND	Orange	5	SGND	Orange
9	RI	Orange-white	9	RI	Orange-white

NOTE:

1. We have received email suggesting that the above pinout looks like DTR from one side is driving into DSR/DCD on the other side - not normally a healthy situation. The emails miss the point that since both ends are DTEs NEITHER should be attempting to drive the DSR/DCD signals. They are essentialy RX only signals on both sides.

RS232 DB9 to DB25 Pinout

Use when connecting a DB9 (e.g. a PC) to a DB25 (e.g. a modem) interface. See the full signal names in the DB9 and DB25 section.

Signal/pin primer

DB9	Signal	DB25
1	DCD	8
2	RD	3
3	TD	2
4	DTR	20
5	SGND	7
6	DSR	6
7	RTS	4
8	CTS	5
9	RI	22

NOTE: Leave all pins not specified above unconnected.

RS232 DB9 to DB25 NULL Modem Pinout

Use when connecting two systems (e.g. PCs) when one has a DB9 interface and the other a DB25 interface without a modem (i.e. back-to-back). See the full signal names in the DB9 and DB25 sections.

Signal/pin primer

DB9	Signal	DB25	Signal
2	RD	2	TD
3	TD	3	RD
4	DTR	6,8	DSR, DCD
6,1	DSR, DCD	20	DTR
7	RTS	5	CTS
8	CTS	4	RTS
5	SGND	7	SGND
9	RI	22	RI

NOTE: Leave all pins not specified above unconnected.

V.35 on DB25 (RS-530-A)

The A and B below refer to each signal pair used in balanced serial interfaces. BEWARE: RS-530 (without the A suffix) is an earlier standard and is wired differently. This is the 530-A pinout spec.

Signal/pin primer

Pin No.	Name	Notes/Description
1	Shield	-
2	Transmit Data (A)	-
3	Received Data (A)	-
4	RTS (A)	Request To Send
5	CTS (A)	Clear To Send
6	DCE Ready	Data Communications Equipment Ready (modem/CSU)
7	Signal Common	-
8	DCD (A)	Data Carrier Detect (a.k.a CD or RLSD)
9	Receiver Signal Element Timing (B)	RX Clock
10	DCD (B)	Data Carrier Detect (a.k.a CD or RLSD)
11	Ext Transmit Clock (B)	-
12	Tramsmit Signal Element Timing (B)	TX CLOCK
13	CTS (B)	Clear to Send
14	Transmitted Data (B)	Secondary Transmit Data
15	Transmit Signal element Timing (A)	TX CLOCK
16	Received Data (B)	RX Data
17	Receiver Signal Element Timing (A)	RX CLOCK
18	Local Loopback	-
19	RTS (B)	Request to Send
20	DTE Ready	Host end
21	Remote Loopback	-
22	RI	Ring Indicator
23	Signal Common	-
24	Ext TX Clock (A)	-
25	Test Mode	-

NOTE: Leave all pins not specified above unconnected.

view - looking into male connector

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